I. THE CONCEPTION OF SPECIES ACCORDING 
TO THE THEORY OF MUTATION. 
i. SYSTEMATIC BIOLOGY AND THE THEORY OF 
MUTATION. 
In every case in which we were able to obtain a deeper 
insight into the nature of the hereditary character of an 
organism by direct observation or experiment we have 
found this character to be of a compound nature. No 
plant transmits its peculiarities to its offspring as an in- 
separable whole, as has been the general view until now. 
On the contrary we have described a long series of phe- 
nomena in which a single character or a smaller or larger 
group of them can be separated from the rest and behave 
in an entirely different way. When new species or vari- 
eties originate, it is not the whole nature of the organism 
that is changed ; on the contrary everything remains in 
a state of rest except at one or two points, and it is only 
to the changes of these points that all the improvement 
is due. In hybridization the two types which sexually 
unite are always alike in the vast majority of their char- 
acters, and the differences between them are limited to 
a few definite units, which, in the simplest cases, can be 
dealt with numericallv. 
j 
The analysis of organisms, therefore, leads us to the 
hypothesis of units, which are in many respects analogous 
to the molecules of the chemist. They are, however, of 
